Ukrainian prime minister Arseny Yatsenyuk accounced today that Russia had “unleashed a new war in Europe” and that “Russian boots are on Ukrainian ground.” According to BBC News, over 1,000 Russian troops (reports are unclear whether they are in official uniform) have crossed into Ukraine and are currently fighting alongside pro-Russian rebels. They now control the Ukrainian border city of Novoazovsk and are attempting to seize Mariupol, a strategic port located on the Sea of Azov — a rebel spokesman has said they will soon “liberate” the port city. The Ukrainian military is now concentrating on strengthening Mariupol’s defenses.

In an interview with Russian television, rebel leader Alexander Zakharchenko claimed that all Russian troops fighting alongside the rebels are doing so as “volunteers,” and are either on leave or retired from active duty.

NATO Brigadier General Niko Tak told BBC News that in the last two weeks, the organization has “detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred [from Russia] to separatist forces in eastern Ukraine.”

“Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favoring the Ukrainian military,” he added.

Ukrainian president Petro Poroshenko abruptly canceled a planned trip to Turkey and convened a meeting of the Ukrainian security council in response to the conflict’s escalation.